If the link is poopy just go to macgamestore.com and select "digital downloads." There are lots of so-so independent games there but also a few "legit" games like the ones you mentioned.

Otherwise there is very little, here and there. The games section at apple.com/downloads offers a wide variety of free demos, of course, and some of these games can be digitally downloaded in their entirety as well (such as Midnight Mansion). If you play Neverwinter Nights, I know that you can successfully download their "premium modules" like Infinite Dungeons (just purchase the Linux .zip file and it will install fine into your OSX version of NWN).